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3.4.1 Atomic force microscopy

The atomic force microscopy (AFM) is a very-high-resolution type of scanning probe microscopy. In the AFM, the sample surface is scanned with a probe consisting of a microscopic tip situated at the end of a cantilever. The bending of the cantilever (contact mode) or damping of its oscillation amplitude (tapping mode) in response to the repulsive or attractive forces between the sample and the tip is monitored by an optical lever. The bending deflection, bending oscillation, and torsion of the cantilever is detected using a laser beam focused onto the end of the cantilever and monitored it reflection by a position sensitive detector (PSD) composed by four-quadrant photodetector. The sample is moved under the tip by the piezoelectric drive in a horizontal plane, while the vertical motion is controlled by a feedback mechanism (figure 3.10). The deflection of the cantilever measures the the force

(a) (b)

Figure 3.10: a) Working principles of AFM, b) Atomic force microscopy Park System NX10 used in this work.

between the tip and the sample. Plotting the deflection of the cantilever versus its position on the sample, a topographic image of the sample is obtained. In a different manner, it is possible to plot the height position of the translation stage. This height is controlled by a feedback loop, which maintains a constant force between tip and sample. To create the height image, the variations of the z-position of the sample during scanning are plotted as a function of the x,y position of the tip. The

3.4 Morphological characterization 47 mechanical properties of cantilevers are characterized by the spring constant kcand

the resonance frequency ω0.

A good cantilever should have a high sensitivity, that can be achieved with low spring constants. Hence, in order to have a large deflection at small force cantilevers should be long and thin.

In AFM applications on soft materials, such as polymers and biological samples, it was found that high tip-to-sample forces in the contact mode often led to mechanical deformation of the surface. For this reason, organic thin films are investigated with the no-contact mode or tapping mode. In tapping mode, tip-to-sample repulsive interactions reduced the oscillation amplitude from the freely oscillating probe (A0).

The oscillation amplitude is kept at the set-point value (Asp) by adjusting the vertical (z-axis) position of the sample with the piezoelectric drive. Such operation modes, called constant force, are the most common in AFM and the cantilever tip hovers about 5-15 nm above the sample surface to detect the attractive van der Waals forces acting between the tip and the sample. In the thin films, the AFM is a useful tools in order to investigate the nanomorphologies of the active layer. Thus AFM images can give qualitative informations about the active layer morphology.

Besides a qualitative analysis, a rough quantitative analysis can be carried out.

The main parameter that allows to compare images of different blend is the Means Square Roughness (RMS). In statistical terms, it is defined in the same ways as the standard deviation. It is calculated as a square root of the mean of the squares of deviations from the mean[93]:

where zi represents the surface height at each data point on the surface profile, z represents the average height of the surface profile, and N is the number of data points.

3.4.2 Scanning Electron Microscope

A scanning electron microscopy (SEM) has been used to study the morphology of the fabric fibers soaked with the conductive solutions of PEDOT:PSS.

SEM is a microscopy which has the aims to provide a topographic image of the objects, at a nanometric scale, rebuilding a signal product by the interaction of an electronic beam with the target. SEM used for the analisys in this work is called

“thermionic emission”, because the beam has reached by the overheating of a filament (generally tungsten) through the use of an electric current; the beam undergoes an

48 3. Materials and methods acceleration to the target by a potential difference (approximately 40 kV) and focused by a system of magnetic lenses. The model used is the Cambridge Stereoscan 360.

An electronic microscopic permit to analyze three kinds of signal: Back Scattered Electrons (BSE), Secondary Electrons (SE) and X-ray. Figure 3.11 a) shows a section of interaction’s volume between the electronic beam and the object to analyze, and the different zones of origin the signals.

(a) (b)

Figure 3.11: a) Section of interaction’s volume between the electronic beam and the sample, b) internal pattern of a SEM.

BSE are characterized by an energy range between 50 eV and the beam’s energy.

Is important point out that those electrons are not emitted by the target, but they are always the original one that has been scattered (elastic scatter) with an angle of almost 180 degrees. BSE signal is proportional to the atomic number of the object to analyze, this permit to map the compositional information of the target. The high energy of electrons allows a penetration around 1 µm, so it returns information about the internal structure loosing the external one.

SE are the product of the interaction between beam and valence’s electrons of external atoms; those are called secondary because they are the product of the inter-action. It is possible detect just the electrons that had been subjected recombination processes, so that came from by the external surface (impossible more than 5 nm).

Their energy, consequently, is lower compared with BSE. SE give an external mor-phology, with an higher resolution of the BSE.

X-rays are product by the loss of energy of atoms after the ionization caused by the impact of primary beam’s electrons with core atom’s one. The ionized core electron create a hole that is fill by an electron of a more external shell; this transition

3.5 Electrical characterization 49